In the sequential coating of motor vehicle bodies using a program-controlled spraying device, such as a painter-robot, it is an important ability to switch the paint flow on and off when the robot reaches accurately defined positions. The speed at which the robot can travel may amount to 1500 mm/sec. Along a straight section of track to be covered at this speed, it must be possible to switch the flow of paint to the spraying device within an accuracy of 4-5 mm. Therefore, in this flow switching, or paint-transfer, section, there must not be delays in excess of 3 ms. However, this 3 ms tolerance is exceeded in practice. The robot control therefore releases adjusting signals to a paint needle-valve with a specific lead time which is predetermined as a function of robot speed.
Paint is usually supplied to the paint needle-valve through conduit lines communicating with a gear-pump or the like and having a return circuit, i.e., bridge, running from the outlet to the inlet of the pump. The purpose of the return circuit is to ensure that the correct operating pressure is maintained at all times at the pump outlet, even when the pump needle-valve is closed. A constant operating pressure is maintained so that the operating pressure does not have to be built up each time the needle-valve is opened. The prior art return circuits contain a pressure sensitive valve which opens automatically in response to pressure in the paint-lines when the paint needle-valve is closed. This pressure sensitive valve closes automatically as soon as the needle-valve is opened. However, the use of these pressure sensitive valves have led to undesirable pressure fluctuations which affect the accuracy with which the jet spray of paint impinged upon the body being coated. Additionally, pressure sensitive valves controlled by the paint pressure respond differently, depending upon the particular rheological properties (i.e. the flow-behaviors) of the paint used.